Determining A Path For Network Traffic Between Nodes In A Parallel Computer

ABSTRACT

Determining a path for network traffic between a source compute node and a destination compute node in a parallel computer including: beginning with an identified group of compute nodes that includes the source compute node and iteratively until an identified group of compute nodes includes the destination compute node: identifying a group of compute nodes, the group of compute nodes having topological network locations included in a predefined topological shape; selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape, and when an identified group of compute nodes includes the destination compute node: selecting a final path for network traffic; and sending a data communications message along the path for network traffic between the source compute node and the destination compute node, the path including, in order of selection, the selected paths and the selected final path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is data processing, or, more specifically,methods, apparatus, and products for determining a path for networktraffic between a source compute node and a destination compute node ina parallel computer.

2. Description of Related Art

The development of the EDVAC computer system of 1948 is often cited asthe beginning of the computer era. Since that time, computer systemshave evolved into extremely complicated devices. Today's computers aremuch more sophisticated than early systems such as the EDVAC. Computersystems typically include a combination of hardware and softwarecomponents, application programs, operating systems, processors, buses,memory, input/output devices, and so on. As advances in semiconductorprocessing and computer architecture push the performance of thecomputer higher and higher, more sophisticated computer software hasevolved to take advantage of the higher performance of the hardware,resulting in computer systems today that are much more powerful thanjust a few years ago.

Parallel computing is an area of computer technology that hasexperienced advances. Parallel computing is the simultaneous executionof the same task (split up and specially adapted) on multiple processorsin order to obtain results faster. Parallel computing is based on thefact that the process of solving a problem usually can be divided intosmaller tasks, which may be carried out simultaneously with somecoordination.

Parallel computers execute parallel algorithms. A parallel algorithm canbe split up to be executed a piece at a time on many differentprocessing devices, and then put back together again at the end to get adata processing result. Some algorithms are easy to divide up intopieces. Splitting up the job of checking all of the numbers from one toa hundred thousand to see which are primes could be done, for example,by assigning a subset of the numbers to each available processor, andthen putting the list of positive results back together. In thisspecification, the multiple processing devices that execute theindividual pieces of a parallel program are referred to as ‘computenodes.’ A parallel computer is composed of compute nodes and otherprocessing nodes as well, including, for example, input/output (‘I/O’)nodes, and service nodes.

Parallel algorithms are valuable because it is faster to perform somekinds of large computing tasks via a parallel algorithm than it is via aserial (non-parallel) algorithm, because of the way modern processorswork. It is far more difficult to construct a computer with a singlefast processor than one with many slow processors with the samethroughput. There are also certain theoretical limits to the potentialspeed of serial processors. On the other hand, every parallel algorithmhas a serial part and so parallel algorithms have a saturation point.After that point adding more processors does not yield any morethroughput but only increases the overhead and cost.

Parallel algorithms are designed also to optimize one more resource thedata communications requirements among the nodes of a parallel computer.There are two ways parallel processors communicate, shared memory ormessage passing. Shared memory processing needs additional locking forthe data and imposes the overhead of additional processor and bus cyclesand also serializes some portion of the algorithm.

Message passing processing uses high-speed data communications networksand message buffers, but this communication adds transfer overhead onthe data communications networks as well as additional memory need formessage buffers and latency in the data communications among nodes.Designs of parallel computers use specially designed data communicationslinks so that the communication overhead will be small but it is theparallel algorithm that decides the volume of the traffic.

Many data communications network architectures are used for messagepassing among nodes in parallel computers. Compute nodes may beorganized in a network as a ‘torus’ or ‘mesh,’ for example. Also,compute nodes may be organized in a network as a tree. A torus networkconnects the nodes in a three-dimensional mesh with wrap around links.Every node is connected to its six neighbors through this torus network,and each node is addressed by its x,y,z coordinate in the mesh. In sucha manner, a torus network lends itself to point to point operations. Ina tree network, the nodes typically are connected into a binary tree:each node has a parent, and two children (although some nodes may onlyhave zero children or one child, depending on the hardwareconfiguration). Although a tree network typically is inefficient inpoint to point communication, a tree network does provide high bandwidthand low latency for certain collective operations, message passingoperations where all compute nodes participate simultaneously, such as,for example, an allgather operation. In computers that use a torus and atree network, the two networks typically are implemented independentlyof one another, with separate routing circuits, separate physical links,and separate message buffers.

During execution of an application in a parallel computer, compute nodesconnected according to a defined network topology may pass many datacommunications messages to other compute nodes in the network. Any delayin data communications increases inefficiency in executing theapplication. There currently exists several typical methods of routingdata communications among compute nodes to reduce delay. Such methodstypically rely on a predetermined set of routing rules or historicalnetwork congestion patterns to determine data communication routes amongcompute nodes. Rules and historical network congestion patterns,however, may not accurately reflect actual network congestion betweennodes in the parallel computer and therefore may not reduce delay indata communications. Readers of skill in the art will recognizetherefore that there exists a need to track network contention amongcompute nodes and use such tracked network contention to select pathsfor network traffic among the compute nodes.

SUMMARY OF THE INVENTION

Methods, apparatus, and products for determining a path for networktraffic between a source compute node and a destination compute node ina parallel computer are disclosed. In embodiments of the presentinvention the source and destination compute nodes are included in anoperational group of compute nodes in a in a point-to-point datacommunications network and each compute node is connected in a networktopology to an adjacent compute node in the point-to-point datacommunications network through a link.

Beginning with an identified group of compute nodes that includes thesource compute node and iteratively until an identified group of computenodes includes the destination compute node, embodiments of the presentinvention include: identifying, by a messaging module of the sourcecompute node, a group of compute nodes, the group of compute nodeshaving topological network locations included in a predefinedtopological shape, each of the compute nodes capable of receiving andforwarding network traffic thereby creating possible paths for networktraffic; selecting, from the predefined topological shape by themessaging module of the source compute node, in dependence upon a globalcontention counter stored on the source compute node, a path for networktraffic between compute nodes having topological network locationsincluded in the predefined topological shape, the selected path fornetwork traffic comprising a portion of the path for network trafficbetween the source compute node and the destination compute node, theglobal contention counter representing network contention currently onall links among the compute nodes in the operational group.

Embodiments of the present invention also include, when an identifiedgroup of compute nodes includes the destination compute node: selecting,from the predefined topological shape by the messaging module of thesource compute node, in dependence upon the global contention counterstored on the source compute node, a final path for network trafficbetween a compute node having a topological network location included inthe predefined topological shape and the destination compute node, theselected final path for network traffic comprising a final portion ofthe path for network traffic between the source compute node and thedestination compute node; and sending, by the messaging module of thesource compute node, a data communications message along the path fornetwork traffic between the source compute node and the destinationcompute node, the path for network traffic between the source computenode and the destination compute node comprising, in order of selection,the selected paths and the selected final path.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescriptions of exemplary embodiments of the invention as illustrated inthe accompanying drawings wherein like reference numbers generallyrepresent like parts of exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary system for determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer according to embodiments of the presentinvention.

FIG. 2 sets forth a block diagram of an exemplary compute node useful ina parallel computer capable of determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer according to embodiments of the present invention.

FIG. 3A illustrates an exemplary Point To Point Adapter useful insystems capable of determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer according to embodiments of the present invention.

FIG. 3B illustrates an exemplary Global Combining Network Adapter usefulin systems capable of determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer according to embodiments of the present invention.

FIG. 4 sets forth a line drawing illustrating an exemplary datacommunications network optimized for point to point operations useful insystems capable of determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer in accordance with embodiments of the present invention.

FIG. 5 sets forth a line drawing illustrating an exemplary datacommunications network optimized for collective operations useful insystems capable of determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer in accordance with embodiments of the present invention.

FIG. 6 sets forth a flow chart illustrating an exemplary method fordetermining a path for network traffic between a source compute node anda destination compute node in a parallel computer according toembodiments of the present invention.

FIG. 7 sets forth a line drawing illustrating an exemplary datacommunications network optimized for point to point operations and apredefined topological shape useful in systems capable of determining apath for network traffic between a source compute node and a destinationcompute node in a parallel computer in accordance with embodiments ofthe present invention.

FIG. 8 sets forth a line drawing illustrating a further exemplary datacommunications network optimized for point to point operations andanother predefined topological shape useful in systems capable ofdetermining a path for network traffic between a source compute node anda destination compute node in a parallel computer in accordance withembodiments of the present invention.

FIG. 9 sets forth a line drawing illustrating a further exemplary datacommunications network optimized for point to point operations and yetanother predefined topological shape useful in systems capable ofdetermining a path for network traffic between a source compute node anda destination compute node in a parallel computer in accordance withembodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary methods, apparatus, and products for determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer in accordance with embodiments of thepresent invention are described with reference to the accompanyingdrawings, beginning with FIG. 1. FIG. 1 illustrates an exemplary systemfor determining a path for network traffic between a source compute nodeand a destination compute node in a parallel computer according toembodiments of the present invention. The system of FIG. 1 includes aparallel computer (100), non-volatile memory for the computer in theform of data storage device (118), an output device for the computer inthe form of printer (120), and an input/output device for the computerin the form of computer terminal (122). Parallel computer (100) in theexample of FIG. 1 includes a plurality of compute nodes (102).

The compute nodes (102) are coupled for data communications by severalindependent data communications networks including a Joint Test ActionGroup (‘JTAG’) network (104), a global combining network (106) which isoptimized for collective operations, and a torus network (108) which isoptimized point to point operations. The global combining network (106)is a data communications network that includes data communications linksconnected to the compute nodes so as to organize the compute nodes as atree. Each data communications network is implemented with datacommunications links among the compute nodes (102). The datacommunications links provide data communications for parallel operationsamong the compute nodes of the parallel computer. The links betweencompute nodes are bi-directional links that are typically implementedusing two separate directional data communications paths.

In addition, the compute nodes (102) of parallel computer are organizedinto at least one operational group (132) of compute nodes forcollective parallel operations on parallel computer (100). Anoperational group of compute nodes is the set of compute nodes uponwhich a collective parallel operation executes. Collective operationsare implemented with data communications among the compute nodes of anoperational group. Collective operations are those functions thatinvolve all the compute nodes of an operational group. A collectiveoperation is an operation, a message-passing computer programinstruction that is executed simultaneously, that is, at approximatelythe same time, by all the compute nodes in an operational group ofcompute nodes. Such an operational group may include all the computenodes in a parallel computer (100) or a subset all the compute nodes.Collective operations are often built around point to point operations.A collective operation requires that all processes on all compute nodeswithin an operational group call the same collective operation withmatching arguments. A ‘broadcast’ is an example of a collectiveoperation for moving data among compute nodes of an operational group. A‘reduce’ operation is an example of a collective operation that executesarithmetic or logical functions on data distributed among the computenodes of an operational group. An operational group may be implementedas, for example, an MPI ‘communicator.’ ‘MPI’ refers to ‘Message PassingInterface,’ a prior art parallel communications library, a module ofcomputer program instructions for data communications on parallelcomputers. Examples of prior-art parallel communications libraries thatmay be improved for use with systems according to embodiments of thepresent invention include MPI and the ‘Parallel Virtual Machine’ (‘PVM’)library. PVM was developed by the University of Tennessee, The Oak RidgeNational Laboratory, and Emory University. MPI is promulgated by the MPIForum, an open group with representatives from many organizations thatdefine and maintain the MPI standard. MPI at the time of this writing isa de facto standard for communication among compute nodes running aparallel program on a distributed memory parallel computer. Thisspecification sometimes uses MPI terminology for ease of explanation,although the use of MPI as such is not a requirement or limitation ofthe present invention.

Some collective operations have a single originating or receivingprocess running on a particular compute node in an operational group.For example, in a ‘broadcast’ collective operation, the process on thecompute node that distributes the data to all the other compute nodes isan originating process. In a ‘gather’ operation, for example, theprocess on the compute node that received all the data from the othercompute nodes is a receiving process. The compute node on which such anoriginating or receiving process runs is referred to as a logical root.

Most collective operations are variations or combinations of four basicoperations: broadcast, gather, scatter, and reduce. The interfaces forthese collective operations are defined in the MPI standards promulgatedby the MPI Forum. Algorithms for executing collective operations,however, are not defined in the MPI standards. In a broadcast operation,all processes specify the same root process, whose buffer contents willbe sent. Processes other than the root specify receive buffers. Afterthe operation, all buffers contain the message from the root process.

In a scatter operation, the logical root divides data on the root intosegments and distributes a different segment to each compute node in theoperational group. In scatter operation, all processes typically specifythe same receive count. The send arguments are only significant to theroot process, whose buffer actually contains sendcount*N elements of agiven data type, where N is the number of processes in the given groupof compute nodes. The send buffer is divided and dispersed to allprocesses (including the process on the logical root). Each compute nodeis assigned a sequential identifier termed a ‘rank.’ After theoperation, the root has sent sendcount data elements to each process inincreasing rank order. Rank 0 receives the first sendcount data elementsfrom the send buffer. Rank 1 receives the second sendcount data elementsfrom the send buffer, and so on.

A gather operation is a many-to-one collective operation that is acomplete reverse of the description of the scatter operation. That is, agather is a many-to-one collective operation in which elements of adatatype are gathered from the ranked compute nodes into a receivebuffer in a root node.

A reduce operation is also a many-to-one collective operation thatincludes an arithmetic or logical function performed on two dataelements. All processes specify the same ‘count’ and the same arithmeticor logical function. After the reduction, all processes have sent countdata elements from computer node send buffers to the root process. In areduction operation, data elements from corresponding send bufferlocations are combined pair-wise by arithmetic or logical operations toyield a single corresponding element in the root process's receivebuffer. Application specific reduction operations can be defined atruntime. Parallel communications libraries may support predefinedoperations. MPI, for example, provides the following pre-definedreduction operations:

-   -   MPI_MAX maximum    -   MPI_MIN minimum    -   MPI_SUM sum    -   MPI_PROD product    -   MPI_LAND logical and    -   MPI_BAND bitwise and    -   MPI_LOR logical or    -   MPI_BOR bitwise or    -   MPI_LXOR logical exclusive or    -   MPI_BXOR bitwise exclusive or

In addition to compute nodes, the parallel computer (100) includesinput/output (‘I/O’) nodes (110, 114) coupled to compute nodes (102)through the global combining network (106). The compute nodes in theparallel computer (100) are partitioned into processing sets such thateach compute node in a processing set is connected for datacommunications to the same I/O node. Each processing set, therefore, iscomposed of one I/O node and a subset of compute nodes (102). The ratiobetween the number of compute nodes to the number of I/O nodes in theentire system typically depends on the hardware configuration for theparallel computer. For example, in some configurations, each processingset may be composed of eight compute nodes and one I/O node. In someother configurations, each processing set may be composed of sixty-fourcompute nodes and one I/O node. Such example are for explanation only,however, and not for limitation. Each I/O nodes provide I/O servicesbetween compute nodes (102) of its processing set and a set of I/Odevices. In the example of FIG. 1, the I/O nodes (110, 114) areconnected for data communications I/O devices (118, 120, 122) throughlocal area network (‘LAN’) (130) implemented using high-speed Ethernet.

The parallel computer (100) of FIG. 1 also includes a service node (116)coupled to the compute nodes through one of the networks (104). Servicenode (116) provides services common to pluralities of compute nodes,administering the configuration of compute nodes, loading programs intothe compute nodes, starting program execution on the compute nodes,retrieving results of program operations on the computer nodes, and soon. Service node (116) runs a service application (124) and communicateswith users (128) through a service application interface (126) that runson computer terminal (122).

As described in more detail below in this specification, the system ofFIG. 1 operates generally for determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer (100) according to embodiments of the presentinvention. A ‘source compute node’ as the term is used in thisspecification refers to a compute node (102) in the operational group(132) from which a data communications message to be transmitted via thepoint to point network (108) originates. A ‘destination compute node’ asthe term is used in this specification refers to a compute node (102) inthe operational group (132) that is the intended recipient or finaldestination of a data communications message originating from a sourcecompute node and traversing the point to point network (108).

In the system of FIG. 1, each compute node (102) is connected to anadjacent compute node in the point to point data communications network(108) through a link. Some links among the compute nodes in the point topoint network (108) may have greater network contention at any giventime than other links forming other paths. ‘Network contention’ as theterm is used in this specification is congestion of data communicationsamong nodes in a parallel computer. Congestion of data communicationsoccurs because each compute node in the system of FIG. 1 includes anetwork buffer that stores data communications for transmission on alink to a neighboring compute node and data communications stored fortransmission in a buffer are delayed until transmitted.

A ‘path’ as the term is used in this specification refers to anaggregation of links and compute nodes through which a datacommunications message travels in the point to point network (108).Because network contention may differ among various links among thecompute nodes (102) in the point to point network (108), a datacommunications message transmitted between a source and destinationcompute node may take different amounts of time to traverse differentpaths between the nodes.

In all embodiments of the present invention described in thisspecification no link in a path for network traffic between a sourcecompute node and a destination compute node, hereafter referred to asthe ‘total path,’ carries a data communications message ‘away’ from thedestination compute node. That is, each link in the total path, andtherefore the entire total path, leads toward the destination computenode. Moreover, it is assumed, for purposes of calculating networkcontention described in detail below, that data communications messagesdo not backtrack or travel across the same link more than once in anyparticular path.

The system of FIG. 1 operates generally for determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer (100) according to embodiments of thepresent invention by, beginning with an identified group of computenodes (102) that includes the source compute node and iteratively untilan identified group of compute nodes includes the destination computenode: identifying, by a messaging module of the source compute node, agroup of compute nodes. Each of the compute nodes in the identifiedgroup of compute nodes is capable of receiving and forwarding networktraffic. Each of the compute nodes in the identified group of computenodes also has a topological network location included in a predefinedtopological shape.

The system of FIG. 1 also operates generally for determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer according to embodiments of the presentinvention by: selecting, from the predefined topological shape by themessaging module of the source compute node, in dependence upon a globalcontention counter stored on the source compute node, a path for networktraffic between compute nodes having topological network locationsincluded in the predefined topological shape. The selected path fornetwork traffic includes a portion of the path for network trafficbetween the source compute node and the destination compute node,hereafter referred to as ‘the total path.’

The point to point network, as mentioned above, may be configuredaccording to various network topologies, such as a torus. A networktopology is generally a description of an arrangement or mapping of theelements, such as links and nodes, of a network, especially the physicaland logical interconnections between nodes. A predefined topologicalshape is a shape within the construct of a network topology comprising agroup of compute nodes, the shape defined by locations of the computenodes within the network topology and their connecting links. Considerfirst, as an example of a topological shape within a network topology, anetwork topology of a mesh network configured as a three dimensionalgrid in which all compute nodes have x,y,z locations within the networktopology. Consider also a topological shape within the network topologyof a three dimensional rectangular prism or a three dimensional pyramidin which each vertex of the shape is defined by a location of a computenode within the topology. Topological shapes may be used for variouspurposes, such as for example, identifying compute nodes located withinthe shape or located outside the shape and identifying paths within theshape or outside the shape. That is, a topological shape is typicallyused only with respect to what it contains or what is does not contain.

A topological shape as the term is used here is described as‘predefined’ because a user typically sets the dimensions of such ashape. Consider again for explanation a topological shape of arectangular prism in a three dimensional grid. A user may set thedimensions of the rectangular prism to three links long, by two linkswide, by three links high. A user may also set a vertex of the shape tobegin at a particular compute node, say a source compute node located ata 0,0,0. As an alternative to defining the edges of a shape, a user mayalso set the dimensions of a topological shape by setting a shape type,such a rectangular prism, and defining opposing vertices of the shape. Auser may, for example, set the opposite vertices of a rectangular prismas the locations of a source compute node and a destination compute nodewithin the topology.

As mentioned above, selecting a path for network traffic between computenodes (102) having topological network locations included in thepredefined topological shape is carried out in dependence upon a globalcontention counter stored on the source compute node. A globalcontention counter represents network contention currently on all linksamong the compute nodes (102) in the operational group (132). That is, aglobal contention counter is a mathematical combination of all values ofeach element of all local contention counters in the operational group(132).

A local contention counter of a compute node represents networkcontention on links among the compute nodes originating from the computenode. A local contention counter may be defined as an array. Consider,for example, that the compute nodes in the system of FIG. 1 areconfigured in a torus network having locations defined by x,y,zcoordinates as described above. In such a torus network a localcontention counter for a compute node may be an array associating a linkdirection and a node location. That is, a local contention counter maydefined as the following array:

-   -   LocalContentionCounter[{link-direction} {compute_node_location}]

In the example of a torus network, a link direction may be x+, x−, y+,y−, z+, and z−, represented in the local contention counter array as 0,1, 2, 3, 4, and 5 respectively. A compute node at location 0,0,0 thattransmits five packets to a compute node located at 3,0,0, on a routeincluding only the x+ axis, may have a local contention counter thatincludes the following elements, assuming no other packets have beentransmitted by the node located at 0,0,0:

-   -   LocalContentionCounter[0,0,0,0]=5    -   LocalContentionCounter[0,1,0,0]=5    -   LocalContentionCounter[0,2,0,0]=5

Each of the elements of the above exemplary local contention counterrepresent packets transmitted on x+ links of compute nodes. The firstelement listed above represents that 5 packets are transmitted on the x+link of the node located at 0,0,0. The second element listed aboverepresents that 5 packets are transmitted on the x+link of the nodelocated at 1,0,0. The third element listed above represents that 5packets are transmitted on the x+ link of the node located at 2,0,0.Although only three elements of a local contention counter are describedhere, readers of skill in the art will recognize that such an array mayinclude an element for each link of each direction of each compute node(102) in the operational group (132).

As mentioned above, identifying a group of compute nodes havingtopological network locations within a predefined topological shape andselecting a path between compute nodes having topological networklocations within the predefined topological shape is carried outiteratively until an identified group of compute nodes includes thedestination compute node. When an identified group of compute nodesincludes the destination compute node, the system of FIG. 1 alsooperates generally for determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer (100) according to embodiments of the present invention byselecting, from the predefined topological shape by the messaging moduleof the source compute node, in dependence upon a global contentioncounter stored on the source compute node, a final path for networktraffic between a compute node having a topological network locationincluded in the predefined topological shape and the destination computenode. The selected final path for network traffic is a final portion ofthe ‘total path,’ that is, the path for network traffic between thesource compute node and the destination compute node.

The system of FIG. 1 also operates generally for determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer (100) according to embodiments of thepresent invention by sending, by the messaging module of the sourcecompute node, a data communications message along the path for networktraffic between the source compute node and the destination computenode, the ‘total path.’ The total path includes, in order of selection,the selected paths and the selected final path.

The arrangement of nodes, networks, and I/O devices making up theexemplary system illustrated in FIG. 1 are for explanation only, not forlimitation of the present invention. Data processing systems capable ofdetermining a path for network traffic between a source compute node anda destination compute node in a parallel computer according toembodiments of the present invention may include additional nodes,networks, devices, and architectures, not shown in FIG. 1, as will occurto those of skill in the art. Although the parallel computer (100) inthe example of FIG. 1 includes sixteen compute nodes (102), readers willnote that parallel computers capable of determining a path for networktraffic between a source compute node and a destination compute node ina parallel computer according to embodiments of the present inventionmay include any number of compute nodes. In addition to Ethernet andJTAG, networks in such data processing systems may support many datacommunications protocols including for example TCP (Transmission ControlProtocol), IP (Internet Protocol), and others as will occur to those ofskill in the art. Various embodiments of the present invention may beimplemented on a variety of hardware platforms in addition to thoseillustrated in FIG. 1.

Determining a path for network traffic between a source compute node anda destination compute node in a parallel computer according toembodiments of the present invention may be generally implemented on aparallel computer that includes a plurality of compute nodes. In fact,such computers may include thousands of such compute nodes. Each computenode is in turn itself a kind of computer composed of one or morecomputer processors (or processing cores), its own computer memory, andits own input/output adapters. For further explanation, therefore, FIG.2 sets forth a block diagram of an exemplary compute node useful in aparallel computer capable of determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer according to embodiments of the present invention. Thecompute node (152) of FIG. 2 includes one or more processing cores (164)as well as random access memory (‘RAM’) (156). The processing cores(164) are connected to RAM (156) through a high-speed memory bus (154)and through a bus adapter (194) and an extension bus (168) to othercomponents of the compute node (152). Stored in RAM (156) is anapplication program (158), a module of computer program instructionsthat carries out parallel, user-level data processing using parallelalgorithms.

Also stored in RAM (156) is a messaging module (160), a library ofcomputer program instructions that carry out parallel communicationsamong compute nodes, including point to point operations as well ascollective operations. Application program (158) executes collectiveoperations by calling software routines in the messaging module (160). Alibrary of parallel communications routines may be developed fromscratch for use in systems according to embodiments of the presentinvention, using a traditional programming language such as the Cprogramming language, and using traditional programming methods to writeparallel communications routines that send and receive data among nodeson two independent data communications networks. Alternatively, existingprior art libraries may be improved to operate according to embodimentsof the present invention. Examples of prior-art parallel communicationslibraries include the ‘Message Passing Interface’ (‘MPI’) library andthe ‘Parallel Virtual Machine’ (‘PVM’) library.

The messaging module (160) of FIG. 2 has been adapted for determining apath for network traffic between a source compute node (152) and adestination compute node in a parallel computer in accordance withembodiments of the present invention. That is, the messaging module(160) in the example of FIG. 2 includes computer program instructionscapable of, beginning with an identified group of compute nodes thatincludes the source compute node and iteratively until an identifiedgroup of compute nodes includes the destination compute node:identifying, by a messaging module (160) of the source compute node(152), a group of compute nodes. Each compute node in the identifiedgroup of compute nodes has a topological network location included in apredefined topological shape and each of the compute nodes is capable ofreceiving and forwarding network traffic thereby creating possible pathsfor network traffic.

The exemplary messaging module (160) of FIG. 2 also includes computerprogram instructions capable of selecting, from the predefinedtopological shape, in dependence upon a global contention counter (616)stored on the source compute node, a path for network traffic betweencompute nodes having topological network locations included in thepredefined topological shape. The global contention counter (616)represents network contention currently on all links among the computenodes in the operational group and may be derived from the localcontention counters (617) of all compute nodes in the operational group.The selected path for network traffic includes a portion of the ‘totalpath,’ the path for network traffic between the source compute node andthe destination compute node.

The exemplary messaging module (160) of FIG. 2 also includes computerprogram instructions capable of, when an identified group of computenodes includes the destination compute node: selecting, from thepredefined topological shape by the messaging module (160) of the sourcecompute node (152), in dependence upon the global contention counter(616) stored on the source compute node (152), a final path for networktraffic between a compute node having a topological network locationincluded in the predefined topological shape and the destination computenode. The selected final path for network traffic in the example of FIG.2 includes a final portion of the ‘total path,’ the path for networktraffic between the source compute node and the destination computenode.

The exemplary messaging module (160) of FIG. 2 also includes computerprogram instructions capable of sending, by the messaging module (160)of the source compute node (152), a data communications message alongthe ‘total path.’ In the example of FIG. 2, the total path includes, inorder of selection, the selected paths and the selected final path.

Although determining a path for network traffic between a source computenode and a destination compute node in a parallel computer is describedwith respect to FIG. 2 as being carried out by a messaging module,readers of skill in the art will recognize that any module havingcomputer program instructions capable of identifying a group of computenodes, selecting a path on which to send a data communications message,and sending the data communications message along the selected path, asdescribed herein, may carry out determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer, and each such module is well within the scope of thepresent invention.

Also stored in RAM (156) is an operating system (162), a module ofcomputer program instructions and routines for an application program'saccess to other resources of the compute node. It is typical for anapplication program and parallel communications library in a computenode of a parallel computer to run a single thread of execution with nouser login and no security issues because the thread is entitled tocomplete access to all resources of the node. The quantity andcomplexity of tasks to be performed by an operating system on a computenode in a parallel computer therefore are smaller and less complex thanthose of an operating system on a serial computer with many threadsrunning simultaneously. In addition, there is no video I/O on thecompute node (152) of FIG. 2, another factor that decreases the demandson the operating system. The operating system may therefore be quitelightweight by comparison with operating systems of general purposecomputers, a pared down version as it were, or an operating systemdeveloped specifically for operations on a particular parallel computer.Operating systems that may usefully be improved, simplified, for use ina compute node include UNIX™, Linux™, Microsoft XP™, AIX™, IBM's i5/OS™,and others as will occur to those of skill in the art.

The exemplary compute node (152) of FIG. 2 includes severalcommunications adapters (172, 176, 180, 188) for implementing datacommunications with other nodes of a parallel computer. Such datacommunications may be carried out serially through RS-232 connections,through external buses such as Universal Serial Bus (‘USB’), throughdata communications networks such as IP networks, and in other ways aswill occur to those of skill in the art. Communications adaptersimplement the hardware level of data communications through which onecomputer sends data communications to another computer, directly orthrough a network. Examples of communications adapters useful in systemsfor determining a path for network traffic between a source compute nodeand a destination compute node in a parallel computer according toembodiments of the present invention include modems for wiredcommunications, Ethernet (IEEE 802.3) adapters for wired networkcommunications, and 802.11b adapters for wireless networkcommunications.

The data communications adapters in the example of FIG. 2 include aGigabit Ethernet adapter (172) that couples example compute node (152)for data communications to a Gigabit Ethernet (174). Gigabit Ethernet isa network transmission standard, defined in the IEEE 802.3 standard,that provides a data rate of 1 billion bits per second (one gigabit).Gigabit Ethernet is a variant of Ethernet that operates over multimodefiber optic cable, single mode fiber optic cable, or unshielded twistedpair.

The data communications adapters in the example of FIG. 2 includes aJTAG Slave circuit (176) that couples example compute node (152) fordata communications to a JTAG Master circuit (178). JTAG is the usualname used for the IEEE 1149.1 standard entitled Standard Test AccessPort and Boundary-Scan Architecture for test access ports used fortesting printed circuit boards using boundary scan. JTAG is so widelyadapted that, at this time, boundary scan is more or less synonymouswith JTAG. JTAG is used not only for printed circuit boards, but alsofor conducting boundary scans of integrated circuits, and is also usefulas a mechanism for debugging embedded systems, providing a convenient“back door” into the system. The example compute node of FIG. 2 may beall three of these: It typically includes one or more integratedcircuits installed on a printed circuit board and may be implemented asan embedded system having its own processor, its own memory, and its ownI/O capability. JTAG boundary scans through JTAG Slave (176) mayefficiently configure processor registers and memory in compute node(152) for use in determining a path for network traffic between a sourcecompute node and a destination compute node in a parallel computeraccording to embodiments of the present invention.

The data communications adapters in the example of FIG. 2 includes aPoint To Point Adapter (180) that couples example compute node (152) fordata communications to a network (108) that is optimal for point topoint message passing operations such as, for example, a networkconfigured as a three-dimensional torus or mesh. Point To Point Adapter(180) provides data communications in six directions on threecommunications axes, x, y, and z, through six bi-directional links: +x(181), −x (182), +y (183), −y (184), +z (185), and −z (186).

The data communications adapters in the example of FIG. 2 includes aGlobal Combining Network Adapter (188) that couples example compute node(152) for data communications to a network (106) that is optimal forcollective message passing operations on a global combining networkconfigured, for example, as a binary tree. The Global Combining NetworkAdapter (188) provides data communications through three bi-directionallinks: two to children nodes (190) and one to a parent node (192).

Example compute node (152) includes two arithmetic logic units (‘ALUs’).ALU (166) is a component of each processing core (164), and a separateALU (170) is dedicated to the exclusive use of Global Combining NetworkAdapter (188) for use in performing the arithmetic and logical functionsof reduction operations. Computer program instructions of a reductionroutine in parallel communications library (160) may latch aninstruction for an arithmetic or logical function into instructionregister (169). When the arithmetic or logical function of a reductionoperation is a ‘sum’ or a ‘logical or,’ for example, Global CombiningNetwork Adapter (188) may execute the arithmetic or logical operation byuse of ALU (166) in processor (164) or, typically much faster, by usededicated ALU (170).

The example compute node (152) of FIG. 2 includes a direct memory access(‘DMA’) controller (195), which is computer hardware for direct memoryaccess and a DMA engine (197), which is computer software for directmemory access. The DMA engine (197) of FIG. 2 is typically stored incomputer memory of the DMA controller (195). Direct memory accessincludes reading and writing to memory of compute nodes with reducedoperational burden on the central processing units (164). A DMA transferessentially copies a block of memory from one location to another,typically from one compute node to another. While the CPU may initiatethe DMA transfer, the CPU does not execute it.

For further explanation, FIG. 3A illustrates an exemplary Point To PointAdapter (180) useful in systems capable of determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer according to embodiments of the presentinvention. Point To Point Adapter (180) is designed for use in a datacommunications network optimized for point to point operations, anetwork that organizes compute nodes in a three-dimensional torus ormesh. Point To Point Adapter (180) in the example of FIG. 3A providesdata communication along an x-axis through four unidirectional datacommunications links, to and from the next node in the −x direction(182) and to and from the next node in the +x direction (181). Point ToPoint Adapter (180) also provides data communication along a y-axisthrough four unidirectional data communications links, to and from thenext node in the −y direction (184) and to and from the next node in the+y direction (183). Point To Point Adapter (180) in FIG. 3A alsoprovides data communication along a z-axis through four unidirectionaldata communications links, to and from the next node in the −z direction(186) and to and from the next node in the +z direction (185).

For further explanation, FIG. 3B illustrates an exemplary GlobalCombining Network Adapter (188) useful in systems capable of determininga path for network traffic between a source compute node and adestination compute node in a parallel computer network according toembodiments of the present invention. Global Combining Network Adapter(188) is designed for use in a network optimized for collectiveoperations, a network that organizes compute nodes of a parallelcomputer in a binary tree. Global Combining Network Adapter (188) in theexample of FIG. 3B provides data communication to and from two childrennodes through four unidirectional data communications links (190).Global Combining Network Adapter (188) also provides data communicationto and from a parent node through two unidirectional data communicationslinks (192).

For further explanation, FIG. 4 sets forth a line drawing illustratingan exemplary data communications network (108) optimized for point topoint operations useful in systems capable of determining a path fornetwork traffic between a source compute node and a destination computenode in a parallel computer in accordance with embodiments of thepresent invention. In the example of FIG. 4, dots represent computenodes (102) of a parallel computer, and the dotted lines between thedots represent data communications links (103) between compute nodes.The data communications links are implemented with point to point datacommunications adapters similar to the one illustrated for example inFIG. 3A, with data communications links on three axes, x, y, and z, andto and fro in six directions +x (181), −x (182), +y (183), −y (184), +z(185), and −z (186). The links and compute nodes are organized by thisdata communications network optimized for point to point operations intoa three dimensional mesh (105). The mesh (105) has wrap-around links oneach axis that connect the outermost compute nodes in the mesh (105) onopposite sides of the mesh (105). These wrap-around links form part of atorus (107). Each compute node in the torus has a location in the torusthat is uniquely specified by a set of x, y, z coordinates. Readers willnote that the wrap-around links in the y and z directions have beenomitted for clarity, but are configured in a similar manner to thewrap-around link illustrated in the x direction. For clarity ofexplanation, the data communications network of FIG. 4 is illustratedwith only 27 compute nodes, but readers will recognize that a datacommunications network optimized for point to point operations for usein determining a path for network traffic between a source compute nodeand a destination compute node in a parallel computer in accordance withembodiments of the present invention may contain only a few computenodes or may contain thousands of compute nodes.

For further explanation, FIG. 5 sets forth a line drawing illustratingan exemplary data communications network (106) optimized for collectiveoperations useful in systems capable of determining a path for networktraffic between a source compute node and a destination compute node ina parallel computer in accordance with embodiments of the presentinvention. The example data communications network of FIG. 5 includesdata communications links connected to the compute nodes so as toorganize the compute nodes as a tree. In the example of FIG. 5, dotsrepresent compute nodes (102) of a parallel computer, and the dottedlines (103) between the dots represent data communications links betweencompute nodes. The data communications links are implemented with globalcombining network adapters similar to the one illustrated for example inFIG. 3B, with each node typically providing data communications to andfrom two children nodes and data communications to and from a parentnode, with some exceptions. Nodes in a binary tree (106) may becharacterized as a physical root node (202), branch nodes (204), andleaf nodes (206). The root node (202) has two children but no parent.The leaf nodes (206) each has a parent, but leaf nodes have no children.The branch nodes (204) each has both a parent and two children. Thelinks and compute nodes are thereby organized by this datacommunications network optimized for collective operations into a binarytree (106). For clarity of explanation, the data communications networkof FIG. 5 is illustrated with only 31 compute nodes, but readers willrecognize that a data communications network optimized for collectiveoperations for use in systems for determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer in accordance with embodiments of the presentinvention may contain only a few compute nodes or may contain thousandsof compute nodes.

In the example of FIG. 5, each node in the tree is assigned a unitidentifier referred to as a ‘rank’ (250). A node's rank uniquelyidentifies the node's location in the tree network for use in both pointto point and collective operations in the tree network. The ranks inthis example are assigned as integers beginning with 0 assigned to theroot node (202), 1 assigned to the first node in the second layer of thetree, 2 assigned to the second node in the second layer of the tree, 3assigned to the first node in the third layer of the tree, 4 assigned tothe second node in the third layer of the tree, and so on. For ease ofillustration, only the ranks of the first three layers of the tree areshown here, but all compute nodes in the tree network are assigned aunique rank.

For further explanation, FIG. 6 sets forth a flow chart illustrating anexemplary method for determining a path for network traffic between asource compute node and a destination compute node in a parallelcomputer according to embodiments of the present invention. In themethod of FIG. 6, the source (602) and destination (612) compute nodesare included in an operational group of compute nodes, the compute nodesare connected for data communications in a point to point datacommunications network, and each compute node is connected in a networktopology to an adjacent compute node in the point to point datacommunications network through a link (103).

Beginning with an identified group (710) of compute nodes that includesthe source compute node (602) and iteratively until an identified group(710) of compute nodes includes the destination compute node (612), themethod of FIG. 6 includes identifying (718), by a messaging module (160)of the source compute node (602), a group of compute nodes (710). In themethod of FIG. 6, each of the compute nodes in the identified group ofcompute nodes (710) has a topological network location included in apredefined topological shape (608) and each of the compute nodes (718)is capable of receiving and forwarding network traffic thereby creatingpossible paths for network traffic.

Identifying (718), by a messaging module (160) of the source computenode (602), a group of compute nodes (710) may be carried out byidentifying compute nodes having topological network locations within apredefined topological shape. As mentioned above, a user typicallydefines the predefined topological shape. A user may also specify thatsuccessive predefined topological shapes, iterations of selected groupsof compute nodes, generally point or aim toward the destination computenode. A user may make such a specification in various ways in dependenceupon the shape of the predefined topological shape. A user may, forexample, specify that a particular vertex of rectangular prism belocated at the network location of the endpoint of a previously selectedpath within the previous rectangular prism.

After identifying a group of compute nodes, the method of FIG. 6continues by selecting (720), from the predefined topological shape(608) by the messaging module (160) of the source compute node (602), independence upon a global contention counter (616) stored on the sourcecompute node (602), a path (714) for network traffic between computenodes having topological network locations included in the predefinedtopological shape (608). In the example of FIG. 6, the selected path(714) for network traffic includes a portion of the path (726) fornetwork traffic between the source compute node (602) and thedestination compute node (612).

Selecting (720), from the predefined topological shape (608) by themessaging module (160) of the source compute node (602), in dependenceupon a global contention counter (616) stored on the source compute node(602), a path (714) for network traffic between compute nodes havingtopological network locations included in the predefined topologicalshape (608) may be carried out by determining total contention for allpaths to each compute node having a topological network locationincluded in the predefined topological shape and selecting a path havingthe lowest total contention.

Selecting (720), from the predefined topological shape (608) by themessaging module (160) of the source compute node (602), in dependenceupon a global contention counter (616) stored on the source compute node(602), a path (714) for network traffic between compute nodes havingtopological network locations included in the predefined topologicalshape (608) may also be carried out by determining a maximum single linkcontention for each path to each compute node having a topologicalnetwork location included in the predefined topological shape andselecting a path having the lowest maximum single link contention.

When an identified group (710) of compute nodes includes the destinationcompute node (612) the method of FIG. 6 continues by selecting (602),from the predefined topological shape (608) by the messaging module(160) of the source compute node (602), in dependence upon the globalcontention counter (616) stored on the source compute node (602), afinal path (728) for network traffic between a compute node having atopological network location included in the predefined topologicalshape (608) and the destination compute node (612). In the method ofFIG. 6, the selected final path (728) for network traffic includes afinal portion of the path (726) for network traffic between the sourcecompute node (602) and the destination compute node (612). Selecting afinal path for network traffic may be carried out exactly as selectingother paths for network traffic as described above, either by selectinga path having the lowest maximum single link contention, selecting apath having the lowest total contention, or in other ways as will occurto readers of skill in the art.

After selecting (602) a final path for network traffic, the method ofFIG. 6 continues by sending (724), by the messaging module of the sourcecompute node (602), a data communications message (725) along the path(726) for network traffic between the source compute node (602) and thedestination compute node (612). In the method of FIG. 6, the ‘total path(726),’ that is, the path (726) for network traffic between the sourcecompute node (602) and the destination compute node (612), includes, inorder of selection, the selected paths (714) and the selected final path(728). Sending (724) a data communications message (725) along the path(726) for network traffic between the source compute node (602) and thedestination compute node (612) may be carried out by embedding in anetwork header of the message, information describing the selected path,so that network communications devices, such as routers, may route themessage through compute nodes according to the information describingthe selected path.

For further explanation consider the data communications networkillustrated in the example of FIG. 7. FIG. 7 sets forth a line drawingillustrating an exemplary data communications network optimized forpoint to point operations and a predefined topological shape (608)useful in systems capable of determining a path for network trafficbetween a source compute node and a destination compute node in aparallel computer in accordance with embodiments of the presentinvention. The data communications network of FIG. 7 is configuredaccording to network topology of a three dimensional grid. Depicted onthe data communications network of FIG. 7 are several predefinedtopological shapes (608), each configured as a rectangular prism. Inaddition to compute nodes located at vertices of the predefinedtopological shapes, the predefined topological shapes (608) of FIG. 7also include all compute nodes having topological network locationswithin the shape and the links that connect compute nodes havingtopological network locations within the shape.

The example of FIG. 7 represents two iterations of identifying a groupof compute nodes and selecting a path for network traffic betweencompute nodes in the identified group. The example of FIG. 7 alsorepresents a selection from the predefined topological shape (608), whenan identified group of compute nodes includes the destination computenode (612), of a final path. The first iteration of identifying a groupof compute nodes and selecting a path for network traffic is representedby a topological shape having a vertex defined by the source computenode (602) located at 0,0,0. The second iteration includes a topologicalshape having opposing vertices defined by a compute node (609) locatedat 1,1,1 and a compute node (610) located at 2,2,2. The selection fromthe predefined topological shape (608) of a final path is depicted inthe example of FIG. 3 includes by a topological shape having opposingvertices defined by the compute node (610) located at 2,2,2 and thedestination compute node (612) located at 3,3,3.

As mentioned above, a user may define such a topological shape byspecifying locations of vertices, number of links making up edges of theshape, and in other ways as will occur to those of skill in the art. Inthe example of FIG. 7, upon each iteration of identifying a group ofcompute nodes, one particular vertex of a topological shape is locatedat an endpoint of a previously selected path (604), or in the case of afirst identification of a group compute nodes, the particular vertex islocated at the source node. Once the particular vertex is defined, andknowing other dimensions of the predefined topological shape asspecified by a user, the group of compute nodes having topologicalnetwork locations within the predefined topological shape (608) may beidentified.

As mentioned above, selecting a path for network traffic between computenodes may be carried out by determining a maximum single link contentionfor each path to each compute node having a topological network locationincluded in the predefined topological shape and selecting a path havingthe lowest maximum single link contention, or by determining totalcontention for all paths to each compute node having a topologicalnetwork location included in the predefined topological shape andselecting a path having the lowest total contention.

Consider as an example of selecting a path having the lowest totalcontention, several possible paths in the exemplary data communicationsnetwork of FIG. 7 between the compute node (610) located at 2,2,2 andthe destination node (612), located at 3,3,3. Although many paths withinthe predefined topological shape exist between the compute node (610)and the destination compute node (612), consider for this example, onlythe path along links (752,753,754), referred to here as the ‘finalpath,’ and the path along links (755,756,757), referred to here as the‘non-selected path.’ Table 1 below describes exemplary networkcontention for links (752-754) in the final path and links (755-757) inthe non-selected path. The network contention depicted for each link inthe exemplary Table 1 is specifically the network contention for acompute node in the direction in which a message would travel from thecompute node to the next compute node in the particular path.

TABLE 1 Network Contention For Links In The Final Path And TheNon-Selected Path Final Path Non-Selected Path Link Contention LinkContention 752 2 755 3 753 1 756 2 754 3 757 4

From the exemplary Table 1, the total network contention for the finalpath, being the sum of each link's (752-754) individual networkcontention, is 6. The total network contention for links (755-757) inthe non-selected path is 9. Between the two paths, a messaging modulemay select a path on which to send a data communications message byselecting the path having the lowest total network contention, that is,the final path.

Consider as an example of selecting a path having the lowest maximumsingle link contention the same final path and non-selected path havingthe same network contention as depicted in the example of Table 1 above.The maximum single link contention for a particular path is the value ofnetwork contention for a link in the path having the highest networkcontention with respect to all other links in the path. In the exemplaryTable 1 above, the final path includes a maximum single link contentionof 3. The non-selected path includes a maximum single link contention of4. In selecting one of the two paths as the path on which to send a datacommunications message between the compute node (610) in the example ofFIG. 7 and the destination compute node (612), the final path may beselected as having the lowest maximum single link contention. Readers ofskill in the art will recognize that in some cases the path having thelowest maximum single link contention may not be the path having thelowest total network contention among all other paths.

A predefined topological shape has been discussed largely in thespecification in exemplary form as a rectangular prism. Readers of skillin the art will recognize however a predefined topological shape inaccordance with embodiments of the present invention may take on manydifferent forms. Consider, therefore, as another example of a predefinedtopological shape (608) within a network topology, the conical shape(608) depicted in the example of FIG. 8. FIG. 8 sets forth a linedrawing illustrating a further exemplary data communications networkoptimized for point to point operations and another predefinedtopological shape (608) useful in systems capable of determining a pathfor network traffic between a source compute node and a destinationcompute node in a parallel computer in accordance with embodiments ofthe present invention. The data communications network of FIG. 8 isconfigured according to network topology of a three dimensional grid.Depicted on the data communications network of FIG. 8 is a singlepredefined topological shapes (608) configured as a cone. The cone (608)in the example of FIG. 7 includes compute nodes falling withinparticular regions (750). A user may define a conical topological shapeby: specifying that the vertex of the cone begin at a source node or anendpoint of a previously selected path, specifying that the axis of thecone be pointed toward the destination compute node, specifying theheight of the axis, and specifying a radius of the base of the cone.

For further explanation consider the data communications networkillustrated in the example of FIG. 9. FIG. 9 sets forth a line drawingillustrating a further exemplary data communications network optimizedfor point to point operations and yet another predefined topologicalshape (608) useful in systems capable of determining a path for networktraffic between a source compute node and a destination compute node ina parallel computer in accordance with embodiments of the presentinvention. The example of FIG. 9 includes three predefined topologicalshapes (608), each defined as a cone. Each cone in the example of FIG. 9includes an axis (808) that points in the direction of the destinationnode. Each cone in the example of FIG. 9 represents an iteration ofidentifying a group of compute nodes, and selecting a path for networktraffic among compute nodes in the identified group. Here, although thepredefined topological shape is not aligned with links such as therectangular prism in the example of FIG. 7, groups of compute nodeshaving topological network locations within the network locations may beidentified according to a user's specification of the cone's dimensions.That is, compute nodes having network locations within a predefinedtopological shape configured as a cone, are compute nodes having networklocations within regions defined by the specification of the cone, suchas, for example, the regions (750) of compute nodes depicted in theexample of FIG. 8.

Exemplary embodiments of the present invention are described largely inthe context of a fully functional computer system for determining a pathfor network traffic between a source compute node and a destinationcompute node in a parallel computer. Readers of skill in the art willrecognize, however, that the present invention also may be embodied in acomputer program product disposed on signal bearing media for use withany suitable data processing system. Such signal bearing media may betransmission media or recordable media for machine-readable information,including magnetic media, optical media, or other suitable media.Examples of recordable media include magnetic disks in hard drives ordiskettes, compact disks for optical drives, magnetic tape, and othersas will occur to those of skill in the art. Examples of transmissionmedia include telephone networks for voice communications and digitaldata communications networks such as, for example, Ethernets™ andnetworks that communicate with the Internet Protocol and the World WideWeb as well as wireless transmission media such as, for example,networks implemented according to the IEEE 802.11 family ofspecifications. Persons skilled in the art will immediately recognizethat any computer system having suitable programming means will becapable of executing the steps of the method of the invention asembodied in a program product. Persons skilled in the art will recognizeimmediately that, although some of the exemplary embodiments describedin this specification are oriented to software installed and executingon computer hardware, nevertheless, alternative embodiments implementedas firmware or as hardware are well within the scope of the presentinvention.

It will be understood from the foregoing description that modificationsand changes may be made in various embodiments of the present inventionwithout departing from its true spirit. The descriptions in thisspecification are for purposes of illustration only and are not to beconstrued in a limiting sense. The scope of the present invention islimited only by the language of the following claims.

1. A method of determining a path for network traffic between a source compute node and a destination compute node in a parallel computer, the source and destination compute nodes included in an operational group of compute nodes in a in a point-to-point data communications network, each compute node connected in a network topology to an adjacent compute node in the point-to-point data communications network through a link, the method comprising: beginning with an identified group of compute nodes that includes the source compute node and iteratively until an identified group of compute nodes includes the destination compute node: identifying a group of compute nodes by a messaging module of the source compute node, wherein each compute node in the group of compute nodes has a topological network location included in a predefined topological shape and each of the compute nodes is capable of receiving and forwarding network traffic thereby creating possible paths for network traffic; selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon a global contention counter stored on the source compute node, a path for network traffic between compute nodes having topological network locations included in the predefined topological shape, the selected path for network traffic comprising a portion of the path for network traffic between the source compute node and the destination compute node, the global contention counter representing network contention currently on all links among the compute nodes in the operational group; and when an identified group of compute nodes includes the destination compute node: selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon the global contention counter stored on the source compute node, a final path for network traffic between a compute node having a topological network location included in the predefined topological shape and the destination compute node, the selected final path for network traffic comprising a final portion of the path for network traffic between the source compute node and the destination compute node; and sending, by the messaging module of the source compute node, a data communications message along the path for network traffic between the source compute node and the destination compute node, the path for network traffic between the source compute node and the destination compute node comprising, in order of selection, the selected paths and the selected final path.
 2. The method of claim 1 further comprising specifying, by a user, dimensions of predefined topological shape.
 3. The method of claim 1 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining total contention for all paths to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest total contention.
 4. The method of claim 1 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining a maximum single link contention for each path to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest maximum single link contention.
 5. The method of claim 1 wherein the predefined topological shape is a cone.
 6. The method of claim 1 wherein the predefined topological shape is a rectangular prism.
 7. An apparatus for determining a path for network traffic between a source compute node and a destination compute node in a parallel computer, the source and destination compute nodes included in an operational group of compute nodes in a in a point-to-point data communications network, each compute node connected in a network topology to an adjacent compute node in the point-to-point data communications network through a link, the apparatus comprising a computer processor, a computer memory operatively coupled to the computer processor, the computer memory having disposed within it computer program instructions capable of: beginning with an identified group of compute nodes that includes the source compute node and iteratively until an identified group of compute nodes includes the destination compute node: identifying a group of compute nodes by a messaging module of the source compute node, wherein each compute node in the group of compute nodes has a topological network location included in a predefined topological shape and each of the compute nodes is capable of receiving and forwarding network traffic thereby creating possible paths for network traffic; selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon a global contention counter stored on the source compute node, a path for network traffic between compute nodes having topological network locations included in the predefined topological shape, the selected path for network traffic comprising a portion of the path for network traffic between the source compute node and the destination compute node, the global contention counter representing network contention currently on all links among the compute nodes in the operational group; and when an identified group of compute nodes includes the destination compute node: selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon the global contention counter stored on the source compute node, a final path for network traffic between a compute node having a topological network location included in the predefined topological shape and the destination compute node, the selected final path for network traffic comprising a final portion of the path for network traffic between the source compute node and the destination compute node; and sending, by the messaging module of the source compute node, a data communications message along the path for network traffic between the source compute node and the destination compute node, the path for network traffic between the source compute node and the destination compute node comprising, in order of selection, the selected paths and the selected final path.
 8. The apparatus of claim 7 further comprising computer program instructions capable of specifying, by a user, dimensions of predefined topological shape.
 9. The apparatus of claim 7 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining total contention for all paths to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest total contention.
 10. The apparatus of claim 7 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining a maximum single link contention for each path to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest maximum single link contention.
 11. The apparatus of claim 7 wherein the predefined topological shape is a cone.
 12. The apparatus of claim 7 wherein the predefined topological shape is a rectangular prism.
 13. A computer program product for determining a path for network traffic between a source compute node and a destination compute node in a parallel computer, the source and destination compute nodes included in an operational group of compute nodes in a in a point-to-point data communications network, each compute node connected in a network topology to an adjacent compute node in the point-to-point data communications network through a link, the computer program product disposed in a computer readable, signal bearing medium, the computer program product comprising computer program instructions capable of: beginning with an identified group of compute nodes that includes the source compute node and iteratively until an identified group of compute nodes includes the destination compute node: identifying a group of compute nodes by a messaging module of the source compute node, wherein each compute node in the group of compute nodes has a topological network location included in a predefined topological shape and each of the compute nodes is capable of receiving and forwarding network traffic thereby creating possible paths for network traffic; selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon a global contention counter stored on the source compute node, a path for network traffic between compute nodes having topological network locations included in the predefined topological shape, the selected path for network traffic comprising a portion of the path for network traffic between the source compute node and the destination compute node, the global contention counter representing network contention currently on all links among the compute nodes in the operational group; and when an identified group of compute nodes includes the destination compute node: selecting, from the predefined topological shape by the messaging module of the source compute node, in dependence upon the global contention counter stored on the source compute node, a final path for network traffic between a compute node having a topological network location included in the predefined topological shape and the destination compute node, the selected final path for network traffic comprising a final portion of the path for network traffic between the source compute node and the destination compute node; and sending, by the messaging module of the source compute node, a data communications message along the path for network traffic between the source compute node and the destination compute node, the path for network traffic between the source compute node and the destination compute node comprising, in order of selection, the selected paths and the selected final path.
 14. The computer program product of claim 13 further comprising computer program instructions capable of specifying, by a user, dimensions of predefined topological shape.
 15. The computer program product of claim 13 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining total contention for all paths to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest total contention.
 16. The computer program product of claim 13 wherein selecting a path for network traffic between compute nodes having topological network locations included in the predefined topological shape further comprises: determining a maximum single link contention for each path to each compute node having a topological network location included in the predefined topological shape; and selecting a path having the lowest maximum single link contention.
 17. The computer program product of claim 13 wherein the predefined topological shape is a cone.
 18. The computer program product of claim 13 wherein the predefined topological shape is a rectangular prism.
 19. The computer program product of claim 13 wherein the signal bearing medium comprises a recordable medium.
 20. The computer program product of claim 13 wherein the signal bearing medium comprises a transmission medium. 